Fuel cells are sources of electric power, in which chemical energy is converted into electric energy by the electrochemical oxidation of an easily oxidizable substance, typically hydrogen by oxygen. Given the low voltage that each individual fuel cell supplies, a large number of fuel cells are generally joined in series, using what are known as interconnectors, in order to increase the electric output, and the fuel cells are joined and sealed in an electrically insulating manner by way of solder glass. These are then referred to as fuel cell stacks or stacks. The individual cell levels, which is to say the ceramic cells comprising the metal interconnector, are also referred to as cassettes. In the stack design, it is necessary to join the individual cassettes along a stack direction not only in an electrically insulated manner, but also in a gas-tight manner. It is necessary to separate the fuel gas ducts of the fuel cell stack in a gas-tight manner from the oxidizing agent chambers of the fuel cell units, and to separate the oxidizing agent ducts of the fuel cell stack from the fuel cell units. The gas supply openings in the cassettes are simultaneously joined to each other by the seals that are applied.
The operating temperature of a high-temperature fuel cell stack (SOFC stack) ranges between 700 and 900° C. A SOFC stack having planar fuel cells typically comprises ceramic cells and metal interconnectors. To this end, the ceramic cell is installed in a metal frame, which in turn is joined to the interconnector. In known fuel cell stacks, sealing and insulating elements made of solder glass or ceramic sealing materials are used in order to bring about the necessary electrical insulating action and the necessary gas tightness.
In general, it is expedient to separate the seal and electrical insulation from each other. The electrical insulation in such a case can be established, for example, by a ceramic element, which is joined to the sheet metal parts by brazing. As a result of the gas-tight brazing bond, the gas supply openings are also sealed at the same time. Here, it is possible to directly braze the ceramic under vacuum to the steel components using active brazing materials.
When joining the stack levels and/or the cassettes to each other, the ceramic cells are usually already integrated in the corresponding joining partners, and thus irreversible damage to the cell is possible during brazing under vacuum due to thermochemical processes (reduction). For this reason, joining in the presence of the cell should always be carried out in an oxidizing atmosphere, such as air.
According to the prior art, it is also possible to use silver-based fillers for this joining in air. These brazing materials allow brazing in air when they contain quantities of additives, such as copper oxide, which promote wetting. Thus, these brazing materials are referred to as RAB (reactive air brazing) materials.
Depending on how the insulating ceramic is produced, it may comprise pores and/or gaps. This is the case, for example, when the ceramic layer is applied onto the metal parts to be insulated by way of a thermal spraying process. Depending on the capillary activity of the brazing material that is used, the material can then penetrate into the gaps that are present and cause short circuits. This is notably the case with silver fillers.
By applying barrier layers to the actual insulating layer, the brazing material can be prevented from penetrating. However, as a result, the coating method becomes more complex than when using only a ceramic insulating layer. It is known from US 2007/0003811 A1 and US 2007/0065707 A1 to employ mixtures of ceramics with metals as the barrier layers and likewise apply them by thermal spraying.
A short circuit, however, can just as well be caused by brazing material escaping the joining gap during the brazing process.
The ceramic cell generally comprises a nickel cermet (the major component being zirconia, the minor component being nickel oxide and/or nickel) and has a relatively uniform relative thermal expansion in the temperature range of RT to 1000° C., which means it has a temperature-independent thermal coefficient of expansion of α=12×10−6 K−1. The sheet metal frame is primarily made of ferritic chromium steel (Fe comprising 22% Cr and other trace elements) and the relative thermal expansion increases with the temperature. The coefficient of expansion increases from α=11×10−6 K−1 at low temperatures to α=14×10−6 K−1 at 1000° C.
The coefficient of expansion of the solder glass generally cannot be exactly matched to the coefficient of expansion of the steel. However, it is known from WO 2006/086037 to reduce the thermal coefficient of expansion of RAB materials, notably by adding aluminum titanate, and thereby better adapt them to the steel that is used.
The disadvantages of the known prior art can be summarized as follows:                The brazing materials (RAB) used result in uncontrolled brazing material discharge and running, which can cause short circuits, for example due to bridging between the metal components.        Relatively large-volume pores and/or pore accumulations develop in the existing joints (RAB) and form cracking points, thereby reducing the mechanical strength of the joint and resulting in leakage (notably in the case of open porosity).        The capillarity and/or reactivity of the brazing material may result in infiltration, notably in the pores and grain boundaries of the ceramic.        Contraction (cohesion) of the brazing material and wetting difficulties can lead to defects in the track and thereby cause leaks.        
As a result, at present, it is often not possible to reproducibly produce tight, insulating joints.